Hypophosphatemia rickets (HYP) is the most common inherited form of rickets in the United States and the most common inherited form of hypophosphatemia. Identification and characterization of specific genetic defects in PEX, the recently identified gene for x-linked (HYP), will be the first goal of this study. The second goal is to identify genes other than PEX that may lead to the HYP phenotype in patients with no mutations in PEX. (This protocol is an ongoing means of identifying mutations in families with hypophosphatemia rickets and is open ended.)